1. Field of the Invention
The present invention relates to an apparatus and method of measuring aspheric surfaces, and more particularly, to an apparatus and method of measuring aspheric surfaces using a hologram.
2. Description of the Related Art
Aspheric lenses are widely used in a variety of products, including large-scale projection display systems and camcorders. The trend toward light-weight, small, and high-picture quality apparatuses has gradually increased the diameter and asphericity of the aspheric lens. The manufacture of the aspheric lens needs high precision and accuracy in shaping the aspheric lens. Accordingly, an apparatus and method to measure the shape of the aspheric lens with high precision using a hologram and interferometer have been developed.
Apparatuses and methods for measuring an aspheric lens using a computer-generated hologram (CGH) were disclosed in U.S. Pat. Nos. 5,737,079 and 5,530,547. The CGH refers to a hologram written by calculating a complex amplitude distribution from a phase distribution of light for an object.
FIG. 1 shows an aspheric surface measuring apparatus disclosed in U.S. Pat. No. 5,737,079. The apparatus includes a light source 1, a beam splitter 2 which alters an optical path, a test plate member 3 having a reference surface 4 on which a CGH 5 for generating reference light WR is written, a test lens 6 having an aspheric surface 7, and an imaging plane 10 on which are formed interference images of the test light WT reflected from the aspheric surface 7 and of the reference light WR. Here, the CGH is written on the reference surface 4 as chrome-on-glass. An aperture plane 8 having an aperture 8a, and a lens 9 are also used to focus on the imaging plane 10.
Light L1 is emitted from the light source 1 and diverges through the beam splitter 2 as light L2. The light L2 proceeds toward the test plate member 3 and the test lens 6. The light L2 is transmitted through the test plate member 3, enters perpendicular to the spherical surface 7 as light L3, and is reflected back along the same optical path as the test light WT. The reference light WR corresponds to the light L2 diffracted at the CGH 5 written on the reference surface 4 of the test plate member 3.
The aspheric surface measuring apparatus has a Fizeau interferometer configuration such that the test plate member 3 is aligned with the other optics to provide a common path for the reference light WR and the test light WT. The apparatus measures the aspheric surface 7 by reading an error in the aspheric surface 7 from a deviation of interference fringes on the imaging plane 10 with respect to a null interference fringe. Null interference fringes show that no interference fringe is formed.
In the aspheric surface measuring apparatus, the test plate member 3 with the CGH 5 needs a high degree of surface precision to reflect the incident light as the reference light WR and the test light WT. Especially, when the test plate member 3 is positioned before the test lens 6, the surface precision of the test plate member 3 is highly important to pass the incident light through the test plate member 3 as the test light WT. However, it is difficult to manufacture the test plate member 3 with such a high degree of surface precision. Another reason for the need of the high-precision test plate member 3 lies in that the test plate member 3 generates the reference light WR.
In the aspheric surface measuring apparatus, the CGH 5 of the test plate member 3 is formed as chrome-on-glass to transmit light. A transparent phase type CGH cannot be used for the CGH 5. Similar to aluminum, chromium provides an opaque silver-like coating. The CGH 5 includes an opaque portion of chromium and a transparent portion of glass. The chrome-on-glass type CGH 5 transmits and reflects the incident light as the test light WT and the reference light WR, respectively. Since the transparent phase type CGH fully transmits the incident light, the reference light WR and test light WT cannot be generated with the transparent type CGH. Accordingly, there is a need to coat the rear of the CGH with aluminum to reflect a portion of the incident light as the test light WT.
When the optical paths of the reference light WR and the test light WT are not common in such an aspheric surface measuring apparatus, measurement errors occur due to environmental factors, such as external vibration. For this reason, the Fizeau interferometer, where the reference light WR and test light WT travel along a common optical path, has been used in the aspheric surface measuring apparatus to minimize the measurement errors.
FIG. 2 shows another conventional CGH aligning and aspheric surface testing apparatus disclosed in U.S. Pat No. 5,530,547. Referring to FIG. 2, an optical mount 17 having a base 11 and a mount plate 13, which is detachably fixed to the base 11, and a frame 15 in which an optical element (not shown) such as a CGH or a CGH null compensator is mounted. The frame 15 and the optical mount 17 are arranged parallel to one another. The frame 15 is releasably coupled to the mount plate 13.
In the CGH aligning apparatus, a spherical test beam is generated by an interference system to create interference fringes after being diffracted by the CGH. The mount plate 13 is adjusted relative to the base 11 to diffract the test beam onto itself, thus producing null interference fringes. The base 11 has screws for adjusting the frame 15 coupled to the mount plate 13. The CGH mounted on the frame 15 is adjusted by the screws until null interference fringes are produced. After the null interference fringes are created, the CGH is removed from the frame 15, and the CGH null compensator is mounted in order to test an aspheric lens.
In the aspheric surface testing apparatus of FIG. 2, since the base 11 and the base plate 13 are manually aligned, it is highly likely that there will be mechanical adjustment errors. Therefore, an aspheric lens having an extreme asphericity cannot be accurately tested with the apparatus. Furthermore, the conventional aspheric surface testing apparatuses cannot be applied to test an extremely aspheric lens with precision due to the limitations of CGH grating spaces.